1. Field of the Invention
The present invention relates to wireless communication systems and, more particularly, to an improved system and method for controlling hand-offs of calls from one wireless network to another.
2. Description of the Related Art
During a call connecting a wireless communication device, such as a cellular telephone, with another telephone, the wireless communication device may cross from the domain of one wireless network to an independent wireless network. Such networks may comprise a company's private cellular telephone network and the public cellular telephone network, for example. Thus, to complete the call, the wireless communication device must be capable of being used in both the company's internal wireless system and in the public system, and accommodation must be made for controlling hand-offs from one network to the other.
Hand-off of a telephone call from the domain of the first wireless network to the domain of the second wireless network may be made via a control link shared between central processing units of the two networks. Using the control link, the two wireless networks are able to negotiate feasibility and operations of hand-off as described, for example, in EIA/TIA interim standard IS42.2-B, "Cellular Radio Telecommunications Intersystem Operations: Intersystem Hand-Off," which is hereby incorporated by reference. While, ideally, hand-offs from one network to another are imperceptible to the user, in practice, hand-offs frequently introduce extraneous clicks and temporary loss of signal.
Frequently a user of the wireless communication device is required to travel from one company site to one or more other company sites, which can cause the wireless communication device to repeatedly switch back and forth between the public and private systems. This can cause unnecessary hand-offs, particularly since typical systems assume that a private wireless system, free of call charges, is preferable to a public system, even though the public systems typically charge a per call access fee. In such systems, a hand-off is always made to the private system, whether or not it is the most efficient in terms of cost or signal quality.
This situation is illustrated more clearly with regard to FIG. 1. In FIG. 1, an exemplary two-network wireless communication system 100 is shown. A private company wireless system is installed in buildings 104, 106 and 108. As illustrated, each building 104, 106, 108 is associated with its own cell or service area 110a, 110b, and 110c, respectively. Overlapping the private wireless system is a public wireless system 102 having multiple cells, only two of which, 102a and 102b, are shown.
As illustrated by the dashed line in FIG. 1, a user can travel from building 104 to building 108, crossing through cell areas 110a, 102a, 110b, 102b and 110c. Assuming that the user begins executing a telephone call in building 104, the user's call will be serviced by the private wireless network so long as he is within region 110a. As soon as the user leaves region 110a, he will be transferred to the public wireless carrier (so long as he is in region 102a). As the user approaches the vicinity of building 106, he will re-enter into the service area of the private wireless network as represented by cell 110b. His call will then be transferred back to the private network. As soon as the user leaves the area surrounding building 106, his call will be transferred from the private network to the public wireless network as he enters region 102b. Finally, as he approaches building 108, his call will be transferred back to the private network as enters region 110c.
By the time the user gets to building 108, he has been transferred four times, each time with a momentary loss of conversation, a change in signal quality and, with every transfer to the public wireless carrier, a new charge per call. Accordingly, a system and apparatus is desired which minimizes the number of unnecessary hand-offs between private and public wireless networks so as to minimize charges and enhance the quality of the call.